The present invention relates to the field of display devices. More specifically, the present invention relates to the field of flat panel display devices utilizing liquid crystal display (LCD) technology.
Flat panel liquid crystal displays (LCDs) are popular display devices for conveying information generated by a computer system. The decreased weight and size of a flat panel display greatly increases its versatility over a cathode ray tube (CRT) display. Flat panel LCD monitors are used today in many applications including the computer component and computer periphery industries where flat panel LCD monitors are an excellent display choice for lap-top computers and other portable electronic devices. Because flat panel LCD technology is improving, more and more flat panel LCD monitors are rapidly replacing CRT displays in other mainstream applications, such as desktop computers, high-end graphics computers, and as televisions and other multi-media monitors.
In flat panel LCD monitors, much like conventional CRT displays, a white pixel is composed of a red, a green and a blue color point or xe2x80x9cspot.xe2x80x9d When each color point of the pixel is excited simultaneously and with the appropriate energy, white can be perceived by the viewer at the pixel screen position. To produce different colors at the pixel, the intensity to which the red, green and blue points are driven is altered in well known fashions. The separate red, green and blue data that corresponds to the color intensities of a particular pixel is called the pixel""s color data. Color data is often called gray scale data. The degree to which different colors can be achieved within a pixel is referred to as gray scale resolution. Gray scale resolution is directly related to the amount of different intensities, or shades, to which each red, green and blue point can be driven.
The method of altering the relative color intensities of the color points across a display screen is called white balance adjustment (also referred to as color balance adjustment, color temperature adjustment, white adjustment, or color balancing). In a display, the xe2x80x9ccolor temperaturexe2x80x9d of white correlates to the relative percentage contributions of its red, green and blue intensity components. In addition, the xe2x80x9ccolor temperaturexe2x80x9d of white correlates to the luminous energy given off by an ideal black body radiating sphere at a particular temperature expressed in degrees Kelvin (K). Relatively high degree K color temperatures represent xe2x80x9cwhitexe2x80x9d having a larger blue contribution (e.g., a xe2x80x9ccoolerxe2x80x9d look). Relatively small degrees K color temperatures represent xe2x80x9cwhitexe2x80x9d having a larger red contribution (e.g., a xe2x80x9cwarmerxe2x80x9d look). Generally, the color temperature of a display screen is adjusted from blue to red while avoiding any yellow-ish or green-ish variations within the CIE chromaticity diagram.
One way to adjust the white balance of a conventional flat panel LCD screen is to alter the gamma and the color look-up tables (LUTs) of the display controller. This method, however, is undesirable because the gray-scale dynamic ranges of the primary colors are severely decreased, causing the display of less stable and less accurate colors.
A novel flat panel LCD screen with dynamically adjustable color balancing system is described in co-pending U.S. patent application Ser. No. 09/087,745, entitled xe2x80x9cA MULTIPLE LIGHT SOURCE COLOR BALANCING SYSTEM WITHIN A LIQUID CRYSTAL FLAT PANEL DISPLAYxe2x80x9d by Daniel E. Evanicky, filed May 29, 1998, and assigned to the present assignee, which is hereby incorporated by reference. The novel flat panel LCD screen includes two light sources of different color temperatures and whose brightness can be independently controlled. One of the light sources is xe2x80x9ccoolerxe2x80x9d (e.g., 7200 K) and another one of the light sources is xe2x80x9cwarmerxe2x80x9d (e.g., 5600 K). By independently adjusting the brightness of these light sources, different color temperature can be achieved. Very accurate colors may be displayed on such flat panel LCD screen.
The novel flat panel LCD screen employs a very low cost calibration evice known as a xe2x80x9cgamma sensorxe2x80x9d or a luminance meter. A method of using the xe2x80x9cgamma sensorxe2x80x9d for calibrating the novel flat panel LCD screen through ratiometric calculations is described in detail in prior-noted co-pending U.S. patent application Ser. No. 09/120,960, entitled xe2x80x9cSYSTEM AND METHOD FOR PROVIDING A WIDE ASPECT RATIO FLAT PANEL DISPLAY MONITOR INDEPENDENT WHITE-BALANCE ADJUSTMENT AND GAMMA CORRECTION CAPABILITIESxe2x80x9d. By combining the luminance measurements of various colors displayed on the flat panel LCD screen and the colorimetric data profile determined at the time of manufacture and stored in the panel""s on-board memory, the white point of the monitor can be calculated and reported to the user using suitable software programs.
Although the chromaticity of the phosphor mixture within each of the light sources generally remains the same, the overall lamp luminance gradually degrades over time depending on usage. As the lamps degrade, the white point range of the system narrows. Additionally, the color profile determined at the time of manufacture may no longer accurately reflect the true color characteristics of the display. As a result, the accuracy of white balance adjustments degrade over time. While the degradation may be slight, it may be unacceptable to color critical applications such as desktop publishing. Thus, for users of those color-critical applications, the useful life of the display is significantly shortened.
Accordingly, what is needed is a method and system for providing users with colorimetric information of a flat panel liquid crystal display that remains accurate over time. What is also needed is a method and system for determining the colorimetric information of a flat panel liquid crystal display that takes luminance degradation into account.
In accordance with embodiments of the present invention, a method and system are disclosed for providing an accurate monitor-specific reference profile for a flat panel LCD monitor over time. The method and system disclosed herein employ a low cost gamma sensor (or luminance meter). Thus, the present method and system are very cost effective.
Embodiments of the present invention are applicable to flat panel LCD monitors with dynamically adjustable color-balancing capabilities. The white balance adjustment mechanisms preferably include the provision of two pairs of light sources of different color temperatures, whose brightness can be independently varied (and distributed through a light distribution mechanism). By varying the brightness (intensity) of the light sources, the color temperature of the display can be altered. Further, the flat panel LCD monitors preferably include memory devices for storing reference profiles that are monitor specific. Further, the memory devices are also configured to be accessible by host computers, display micro-controller units or gamma sensors (e.g. luminance sensors) such that the reference profiles can be accessed during color calibration.
According to an embodiment of the present invention, the reference profiles are updated at reasonable intervals by measuring the luminance of the primary colors (Red, Green and Blue) at various brightness settings with a gamma sensor (or luminance sensor). The luminance data are then used for correcting the extensive colorimetric reference profile of the display to account for luminance degradation. By using an updated colorimetric reference profile, accurate color information of the display can be provided to the users. Because a low cost gamma sensor is used for updating the color reference profile, the embodiments of the present invention are very cost effective.
Embodiments of the present invention include the above and further include a method for updating a colorimetric reference profile for a flat panel liquid crystal display (LCD) monitor including a first light source having a first color temperature and a second light source having a second color temperature. The present method includes the steps of: displaying a plurality of monochromatic windows one at a time on the flat panel LCD monitor at a plurality of brightness settings; synchronous with the step of displaying, measuring luminance values of the displayed images with a gamma sensor; accessing a previously stored colorimetric reference profile of the flat panel LCD monitor; and, determining an updated colorimetric reference profile for the flat panel LCD monitor based on the luminance values of those previously stored and the current luminance measurements. In the present embodiment, the updated colorimetric reference profile accurately reflects effects of luminance degradation of the first light source and the second light source.